The present invention relates an improved apparatus in the art of machine spraydeveloping of lithographic printing plates or cleaning, etching and chemical processing flat sheets.
In the field of preparing lithographic printing plates, light sensitive coatings are placed upon a support, usually a flexible, thin metal sheet, thus creating a presensitized printing plate. The plate is subjected to imagewise exposure through a mask which renders the photosensitive substance insoluble in a suitably chosen developer, if it is negative working, and soluble in the developer, if the plate is positive working. The action of the developer, in the case of lithographic plate, is to remove the non-image portions of the plate coating completely so that no trace of it remains on the metal support. The metal support, so revealed, is now free of coating and will no longer accept ink. This sharp discrimination between image and non-image areas on the plate is vital to success in the preparation of the lithographic plate.
Exposed lithographic plates may be developed by hand or by machine. If developed by hand, developer is poured both upon the plate and upon a soft sponge or pad and rubbed therewith upon the surface of the plate until, by inspection, the non-image areas are completely removed. Care must be taken to avoid any damage to the image by excessive rubbing, or by the use of an overly active chemical developer.
The disadvantages of hand development of offset printing plates are numerous. The process is slow and expensive. Uniformity of pressure in applying the developing solution to remove the undesired coating is almost impossible to attain and exposure to the developing solution is uneven, Thus, defective printing often results from an inadequate development or underdevelopment of an area of from applying varying pressure by hand, which may adversely affect the desired printing image. Drying of the developer on certain portions of the plate before it can be rubbed in to remove the undesired coating also may leave a residue on the plate. A further difficulty with the hand method is in the application of the developing solution. It is presently the practice of the craftman to pour a quantity of developer onto a developing sponge or pad and onto the center of the plate, which quantity is supposedly sufficient to process the plate, and the craftman then works his sponge from the "reservoir." This procedure may lead to a high degree of contamination of the processing fluids by the removed photosensitive coating as well as a change in the chemistry of the fluid because of evaporation, which will, in turn, either reduce the efficiency of the chemistry resulting in incomplete removal of the undesired coating in highly critical areas of halftones, or increase its potency resulting in image attack.
Further, the development of a two-sided plate by developing one side at a time in a sink and turning the plate over can result in damage to the first developed side by pieces of contaminated material in the sink being forced back into the image or non-image areas of the first developed side when the second side is developed.
These defects may not become apparent until the plate is clamped into a printing press and the expense of developing the plate has been incurred. All of these problems become more critical with increasing plate sizes.
When many plates are to be developed, machinery is used which will develop the plate by applying developer mechanically.
Automatic processors are available which overcome the difficulties associated with the conventional hand development of such plates by providing an enclosed developer apparatus which includes a receiving station for receiving a printing plate, continuous transport system, a developing station in which the developer is applied to both side of the plate simultaneously and evenly over the complete surface of both sides, including a pre-soak section to soften the coating, a rubbing or scrubbing section consisting of a free-floating assembly of a set of cleaning brushes and a set of velour type cloth coated scrubber pads mounted one above and one below the plate in such a manner as to facilitate easy entrance of the plate between the cleaning brushes and scrubber pads. The entire cleaning and scrubbing assembly is further capable of a reciprocating movement traversely across the surface of the plate, each half in a direction opposite to the other. When no plate is in the developing system, the top and bottom cleaning and scrubbing devices rub against each other. Attached to the end of the assembly are furthermore squeegee means which contact the following transport squeegee rollers for reasons described below.
Prior to entering the cleaning and scrubbing section as just described, the developer is applied onto both sides of the plate in a unique pre-soak chamber. While the plate is being moved forward through the pre-soak chamber, the coating on the non-image areas is softened, enabling the following cleaning and scrubbing assembly to effectively clean or remove all unwanted coating even in the smallest areas. This assures a highest quality image. Developer is further applied to the plate continuously in the cleaning and scrubbing section, both through the actual cleaning brushes and between the scrubber pads. The plates then enter a wash water stage for the removal of excess developer and removed excess non-image plate materials. In some existing machines, developer is metered to the plate through tubes and spread about by sponges or brushes which also serve to separate loosened non-image particles from the plate support. In the next stage, within the machine, the spent developer and separated non-image coating are rinsed from the plate, usually with water, supplied through tubes. In a final stage, the rinsed, moist surface is covered with a gum solution and any excess thereof is removed, thereby delivering a plate ready for the press or for storage. The gum solution is also metered to the plate through tubes.
In some cases, these processing solutions are pumped through spray nozzles which are directed upon the plate surface. In further refinements, the developer and gum solution overflows are returned to reservoirs from which they are again pumped through the spray nozzles.
In the case of developers for certain positive working plates, recirculation is not feasible due to the aeration of the developer. In this case, to avoid aeration, only fresh developer may be applied to the plate.
With regard to developing machinery utilized in the graphic arts, other objects are:
to reduce the needed quantities of valuable developers and finishing agents; PA1 to increase the speed of operation; PA1 to increase the completeness of development; PA1 to increase the discrimination between image and non-image areas; and PA1 to permit the use of less aggressive developer, and thus decrease damage to the image PA1 a. a plate processor having an enclosed housing capable of containing wash water therein, said processor having a plate inlet and a plate outlet and means for applying wash water to a plate between said inlet and said outlet; and PA1 b. a wash water reservoir external to said housing; and PA1 c. means for pumping wash water from the reservoir to the wash water applying means of the processor; and PA1 d. means for collecting wash water from the wash water applying means of the processor and returning it to the reservoir; and PA1 e. means for continuously and automatically measuring the pH of the wash water in the reservoir; and PA1 f. injection means responsive to said pH measuring means capable of automatically adding a pH adjusting composition to the wash water in the reservoir at a predetermined pH. PA1 a. providing a wash water reservoir external to said processor and pumping wash water from the reservoir to the processor, and collecting used wash water from the processor and returning it to said reservoir; and PA1 b. automatically and continuously measuring the pH of the wash water in the reservoir; and PA1 c. automatically injecting a pH adjusting composition into the wash water in the reservoir and maintaining the pH of the wash water in a predetermined range.
In machinery for the processing of exposed lithographic printing plates, the plate is usually transported under spray heads through which developing solutions, rinses and finishing solutions, as determined by the needs of the individual plate, are sequentially dispensed. Appropriate time intervals are provided for the action of each solution, optionally assisted by soft non-scratching brushes, scrubbing pads, squeegees and the like.
Plate processor are per se well known in the art as exemplified by U.S. Pat Nos. 4,239,368; 3,995,343; 3,937,175; 3,738,249; 3,809,105; 3,771,428; 4,081,577; and 4,091,404 which are incorporated herein by reference.
A problem with the heretofore known processors is their excessive use of wash water. Indeed, it is the experience of large press shops that the annual cost of the wash water alone which is pumped through the processor, vastly exceeds the cost of the processor equipment itself. If a wash water is used on a "one shot" basis it is discarded after the single use although it still has substantial residull washing ability. As a result of this capacity, prnnters desire to use wash water in processing machines again and again to gain greater economics. However, as the non-image areas of the photographic surface of the plate are removed, they are carried away and contaminate the water. Processors frequently have filters which remove flaked off coating particles to prevent a redeposit of these coatng flakes onto the plate. Such a processor is shown in U.S. Pat. No. 4,239,368. However, over time, the non-filterable portion of the liquid wash water is itself contaminated with unfiltered components. These are typically alkaline coating components which unacceptably raise the pH of the solution. In such an event, the solution is either discarded, or upon periodic monitoring of pH by the printer, an amount of fresh water is added. These, however, are short-term fixes to the problem because the solution is not continuously monitored and because the addition of such neutralizing acids may restore the pH to neutral but the conductivity of the solution increases to such an extent that the wash water quickly becomes unworkable. The present invention provides an improved processor for lithographic printing plates wherein pH and conductivity are continuously monitored and pH adjusting solution is automatically added as needed while maintaining wash water conductivity within an acceptable working range.